Wireless data networks, such as those using the well-known Institute of Electrical and Eectronics Engineers (IEEE) 802.11 standards, are widely used to connect devices to enterprise networks (such as LANs or WANs) or to the Internet. The IEEE 802.11 standards are designed primarily for the efficient transmission of data. With the increased popularity of voice-over-IP (VoIP) and other real-time applications such as the NetMeeting™ application (from Microsoft Corp.), the need to support these real-time applications over wireless networks, including IEEE 802.11 networks, has become increasingly important.
In a conventional implementation (FIG. 1), an IEEE 802.11 wireless network includes an access point (AP) connected to a wired network. The network also includes a number of devices (or clients), each equipped with an IEEE 802.11 interface. The AP and clients share one wireless channel to exchange packets generated by various applications. The wireless channel may suffer from various impairments such as fading, interference, and attenuation. The wireless transceivers adjust their transmission rate to maintain a low packet error rate. Thus, the AP and clients transmit more slowly when their respective channels are degraded.
The media access control (MAC) protocol regulates the access to the wireless channel. The basic MAC mechanism in IEEE 802.11 networks is collision avoidance. Before transmitting, each device waits for the channel to be idle and then transmits a packet. When a device successfully receives a packet, it immediately sends back an acknowledgment. If a device transmits a packet but does not receive an acknowledgment, the device may determine that its transmission collided with that of another device. The device then calculates a random time and waits until it detects that the channel has been idle for that amount of time before it attempts to transmit its packet again. As the number of active devices increases, so do the likelihood of collision and the typical delay until successful transmission. This delay further increases if some of the devices transmit large packets and if the channel suffers impairments.
The IEEE 802.11e protocol attempts to address limitations of the standard IEEE 802.11 networks. IEEE 802.11e provides differentiated access to the wireless channel by giving some form of favored treatment to certain classes of traffic. The IEEE 802.11e protocol groups packets into different classes and provides differentiated channel access for the different classes. Using that protocol, a device that tries to send a packet of a more urgent class is required to wait a shorter channel idle time before it attempts to transmit; and in the event of a collision, the device chooses a random time that tends to be shorter than the idle time for less urgent classes. Such modifications of the MAC may improve the ability of the network to support critical applications. However, the IEEE 802.11e protocol still suffers from undesirable limitations.
Although IEEE 802.11e modifications improve the ability of the 802.11 networks to support VoIP applications, these improvements still fall short of the characteristics needed in many situations. First, these variations are less effective when a number of clients do not support the modified protocol and are standard 802.11b and/or 802.11g devices. Second, the MAC parameters of IEEE 802.11e indirectly determine the number of acceptable VoIP connections that the network can support but do not enable the network manager to modify the desired operating point. Third, as in any 802.11 network, excessive delays occur when too many devices compete for the channel.